Reap unused accounts

[eevee]

I am so frowny about people who register for a thing, use it once or not at all, and then bail forever. This won't be a problem for quite a while, but I would really really like to have something in mind when the time comes.

[epii]

Idle thoughts:

The "not at all" case is way simpler than "once" (or "few"). As a first step, perhaps accounts should be reaped if they:

1. Have no community-visible activity (art, comments, watching, rating);
2. Have existed for at least a fortnight; and
3. Have not been logged in within the smallest of either:
   • 3 months; or
   • The last 80% of the period since the account's creation.

Or something similar with tweaked numbers.

I don't think we can reasonably ever delete an account that has uploaded art (provided the submissions fit whatever ToS we end up with). Accounts that have only commented, watched, rated, etc. are grey. I'm inclined to not reap accounts that have commented (at least not without careful staff review). The number of interactions must obviously be accounted for. Maybe even a simple rule like "fewer than 10 interactions" would be sufficient (and maybe any interactions bumps the "not been logged in" max value to 6 months or something).

Obviously none of the above will hinder malicious name-sitting, but I'm not sure that's even likely to be a widespread problem.